In continuous binary array (or digital) ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array. The ink discharges from the orifices in filaments, which break into droplet streams. Printing is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other drops strike a drop catcher device. As one example of a continuous binary array printer, reference is made to U.S. Reissue Pat. No. 34,932.
In binary array printing, jet straightness and wetting of the nozzle plate in the area of the orifices is vitally important. The slightest deposit, or a non-uniform wetting on the orifice plate may cause excessive raggedness of the jets and results in poor print quality. Inks for use specifically in binary array printers have been described, for example, in U.S. Pat. No. 5,601,639.
Printer runability is a primary concern during the formulation of binary array inks and severely limits which raw materials can be used in the inks. Issues influencing runability of an ink in a binary array printer include, but are not limited to: stable jet formation, print degradation, jet straightness, shear stress stability, corrosivity, and sensitivity to environmental conditions.
The formation of stable jet break off is critical to the operation of a continuous ink jet printer, especially binary array printers. Variations of jet formation will lead to uncontrolled jets. In addition to designs of the printer, the viscosity of the ink at the time of printing controls this issue. Print degradation is a loss of print quality, often due to ink build up on the orifice face. The presence of some salts and other soluble contaminants influence this issue.
Jet straightness is critical to operation of a binary array printer. Deviation of one jet from its expected path can cause that jet to not be controlled and cause a print failure. This control issue is more significant in the operation of binary array continuous ink jet printheads than in corresponding non-array continuous ink jet printheads. Particulate and insoluble contaminants in or around the nozzle orifice are factors in influencing this issue. High purity and solubility are required for all components of the ink.
The stability of the ink to shear forces is required because the ink is re-circulated through the printer by a gear pump. If the ink is unstable to shear forces, extended use in the printer will be curtailed. Instability to shear forces may cause the formation of insoluble material and lead to printer failure.
Similarly, if the ink is corrosive to the printer components, extended printer usability will be reduced. Selection of conductivity agents and dyes is strongly influenced by this issue.
If the ink is extremely sensitive to the ambient conditions, such as relative humidity, the properties of the ink will change and limit the use in the printer. Insoluble material may also be formed if the ink is too sensitive to the operating conditions of the printer.
Further, it is well-recognized that solvent evaporation occurs during continuous ink jet printing, requiring the use of make-up fluid to maintain a desired ink composition.
Because of all of the foregoing considerations, it is difficult to provide a suitable ink that functions satisfactorily in a binary array ink jet printing system.